In a planar device such as a Metal Oxide Semiconductor Field Effect Transistors (MOSFET), a source, a gate and a drain are arranged in a direction substantially parallel to a substrate surface. Due to such an arrangement, the planar device is difficult to be further scaled down. In contrast, in a vertical device, a source, a gate and a drain are arranged in a direction substantially perpendicular to a substrate surface. As a result, the vertical device is easier to be scaled down compared to the planar device.
However, it is difficult for the vertical device, especially one whose channel includes a single-crystalline material, to control its gate length. On the other hand, the channel of a poly-crystalline material will have its resistance significantly increased compared to the single-crystalline material. Therefore, it is difficult to stack multiple vertical devices, because this will result in too high resistance.